The circadian timing system in advanced multicellular animals plays a major role in the control of behavioral and physiological functions. There is increasing evidence that this circadian system is composed of multiple potentially independent oscillating units, with a probably hierarchical organization, which are synchronized with one another through circadian rhythms in hormonal and nervous mediators. This research project is concerned with the central neural components of this circadian timing system in the squirrel monkey (Saimiri sciureus). Utilizing a trained chair-acclimatized, unanesthesized squirrel monkey preparation which we have developed in this laboratory, we propose to investigate the role played by these neural components in the external and internal synchronization of behavioral and physiological parameters in the circadian system. Particular attention will be payed to the function of the suprachiasmatic nucleus (SCN) of the hypothalamus. Studies of SCN-lesioned animals will be undertaken to determine whether the SCN plays a central pacemaker role in the circadian system and whether it is under the phase control of zeitgebers other than light-dark cycles. The squirrel monkey preparation is particularly suitable for these studies because multiple diverse behavioral and physiological functions can be simultaneously monitored for many successive circadian cycles, and a considerable body of previous experimental data has been accumulated on the normal functioning of the circadian timing system in this species. By pursuing these studies, we aim to establish in a primate the neural control of the circadian system which plays an essential timekeeping role in higher animals, and has many important implications for clinical psychology and medicine.